587 



parts of undried hay and corn could raise one pound of water 0.682 F., 

 which from previous experiments he knew was equivalent to 557 foot- 

 pounds. From these results it appeared that one-quarter of the whole 

 amount of energy generated by combustion of the food could be converted 

 into useful mechanical work, the remaining three-quarters being required 

 to keep up the animal heat, etc. (Scoresby and Joule). 



Since these first measurements by Joule many estimates have been made 

 of the mechanical efficiency of various kinds of muscular work both in ani- 

 mals and men. It turns out that the efficiency depends upon the type of 

 work performed, i. e., the particular muscles used, the training, the speed 

 with which the work is done, and the kind of food which sustains the 

 metabolism. 



It is necessary at this point to distinguish between gross efficiency and 

 net efficiency. The former term is found by dividing the mechanical 

 work in terms of heat by the total metabolism of the time ; while net effi- 

 ciency, the more exact term from the standpoint of bio-physics, is found 

 by dividing the heat equivalent of the mechanical work by the extra metab- 

 olism' due to the work accomplished. This is found of course by subtract- 

 ing the basal or resting metabolism from the total work metabolism. Un- 

 less otherwise specified the figures used in this chapter refer to net effi- 

 ciency. 



From data obtained by Lavoisier upon his assistant, Seguin, whose 

 oxygen absorption was measured during rest and while working a treddle, 

 Benedict and Cathcart have calculated that at most an efficiency (net) 

 of 7.7 per cent can be made out. This work of Lavoisier represents the 

 earliest collection of data from which the efficiency of human muscles can 

 be computed. Helmholtz presented the next in order historically when 

 he assembled data from the work of Edward Smith, of Dulong and of 

 Despretz, which according to his reckoning showed a gross efficiency of 

 approximately 20 per cent. Amar cites experiments by Hirn done in 

 1857 which, assuming that the total heat elimination was correctly meas- 

 ured, demonstrate an efficiency of about the same amount. Other im- 

 portant workers of the French school in this field are Laulanie(d) and 

 Chauveau(a.). The former studied especially the influence of speed upon 

 efficiency. He found in experiments upon himself that so long as the rate 

 was constant, turning a wheel with a brake attachment 5, 10 or 15 minutes 

 gave the same efficiency, but when the load and speed were varied the 

 efficiency varied from 9 to 23 per cent. The load varied from 1 to 15 kilo- 

 grams and the speed from 1.49 to 0.13 meter per second. The highest 

 efficiency was shown with a moderate load (4 kilograms) and a moderate 

 speed (0.61 meter per second^). This accords with everyday experience. 



Chauveau's observations made upon his assistant, Tissot, were directed 

 especially to the question of the kind of foodstuffs which supports mus- 

 cular work. They will be referred to later. 



